Precision potentiometers



y 11, 367 N. A. DOUGLAS PRECISION POTENTIOMETERS Filed Jan. 5, 1964 ,m u R ow H To m2 v 6 m w 1 M United States atent 3,331,047 PRECISION POTENTIOMETERS Nicholas A. Douglas, Bronx, N.Y., assignor to Litton Industries, Inc., Beverly Hills, Calif. Filed Jan. 3, 1964, Ser. No. 335,497 2 Claims. (Cl. 338143) This invention relates to precision otentiometers and a method of making the same.

Precision potentiometers of the multiturn type include closely wound coils of resistance wire or convolutions of resistance material bonded to an insulating form or support. A movable contact member is provided to make slidable electrical contact with the resistance element. In a compact multiturn potentiometer, instead of employing the resistance element to guide the contact member, a contact guide member or track adjacent the resistance element is advantageous since it permits the use of a minute contact wire or whisker which provides uniform contact resistance with the resistance element, and the guide track positions the slider which carries the contact wire.

One form of contact guide member which-has been extensively used in practice consists of a plastic-coated Wire that is wound on and supported by the convolutions of the resistance element, thus providing a groove to receive the movable contact, the bot-tom of which is bounded by the surface of the resistance element. The use of a plastic-coated wire guide places limitations upon the temperature to which the potentiometer may be subjected in service since a high ambient temperature will soften or destroy the plastic coating. Furthermore it has been found that, after a protracted period involving extensive movement or many adjustments of the movable contact member, the rubbing of the contact or slider lug against the surface of the guide member causes wear. The resulting deposit of insulating particles or dust on the surface of the resistance element increases the contact resistance where the dust is deposited, and so-called electrical noise is produced in the output voltage characteristics of the potentiometer.

The object of the invention is to overcome these difficulties and provide a unit having improved characteristics, longer life and the ability to withstand higher temperatures.

In accordance with the invention, a multiturn precision potentiometer is provided with a glass insulating member of novel character. For example, when used as a contact guide member, the insulating member of glass or glasslike material, e.g., quartz, may be formed as a rod disposed adjacent the contact surface of the resistance element and forming a guide track for the movable contact member. The invention also embraces the concept involving a method of making a potentiometer of the abovementioned type which includes the step of mounting a preformed glass coil or helix on the convolutions of the resistance coil or element.

While the construction embodying the invention has been found particularly advantageous as a guide member for the movable contact, the invention is not limited to this use. In a broader aspect, the invention may be utilized to obtain an insulating member of glass or glass-like material for various purposes where the high-temperature properties or other characteristics of such materials are necessary or desirable. For such purposes, a preformed glass element of the desired shape for example may be incorporated in a potentiometer to provide a permanent insulating or spacing member.

The novel features which are believed to be characteristic of the invention, together with further objects and advantages thereof, will be apparent from the following ice description of an illustrative embodiment thereof shown in the accompanying drawings, wherein FIG. 1 is a side elevation, partly in section, of a multiturn potentiometer employing a glass guide member for the movable contact;

FIG. 2 is a view illustrating the method of assembly of the resistance element and guide member of the potentiometer shown in FIG. 1; and

FIG. 3 is a detail in cross-section to an enlarged scale.

Referring to FIG. 1 of the drawings, 'a multiturn potentiometer 10 is shown which comprises a ceramic coil or coil form 11 on which is wound a helical resistance element 12, said coil form being supported on a base or cover plate 14. A rotatable input shaft 15 attached to a slider assembly 17 is mounted on the cover plate 14. The slider assembly includes the offset extension 17a constituting a slider arm parallel to the shaft 15 for supporting a slider 18 which carries an adjustable contact 19 engaging the resistance element 12. As shown by way of example, the potentiometer is of the type disclosed and claimed in the prior patent of James W. Weidenman et al. No. 2,871,326, granted Jan. 27, 1959. In that patent the movement of the slider across the width of the resistance element is effected by a guide track consisting of an insulating wire that is wound on the closely spaced turns of the resistance element. The movable contact engages the guide track and this construction may also be employed in the present case if desired but, as shown, the slider 18 is provided with a separate guide lug 22 to effect the transverse feeding of the contact member 19 without any engagement between the contact member and the guide track 21. This permits the use of a contact member 19 in the form of a thin, flexible wire or whisker. While this construction has proven highly satisfactory in actual service, any suitable contact member and associated slider 18 may be employed.

In accordance with the invention, the helical guide member or track 21 is made of glass or glass-like material, such as borosilicate glass (Pyrex) having desired strength and size-stability characteristics under high temperatures, and a smooth track surface.

In order to form a glass track 21 resting on or contiguous to the convolutions of the resistance element 12, the preferred method involves the use of a preformed coil of glass rod as shown in FIG. 2, said rod having a predetermined diameter which is correlated with the spacing between the turns of the resistance element. It has been found satisfactory to utilize a preformed coil of borosilicate glass having an internal diameter 28 which is approximately equal to the outside diameter of coil form 11. However the internal diameter of the coil may be formed either smaller or larger than the dimension 28, since the elasticity and tensile strength of the glass permits the turns thereof to be expanded or contracted to fit over the resistance element in the position shown in FIG. 1. With this arrangement, the glass helix is supported by the resistance element 12 and forms an elongated guide or boundary surface extending along the length thereof. The helical glass coil may be formed in any desired manner, as for example by winding a glass rod which has been heated to the softening point upon a mandrel of the proper outside diameter with a pitch or spacing between the turns of the coil, as indicated by the dimension 29, which is preferably equal to the pitch of the resistance element 12. As shown in FIG. 2, the ends of the resistance element 12 are clamped on the core 11 by clamping members 23 and 24. Other clamping members 25 and 26 may be employed to hold the glass helix 21 in position, with a pad or cushioning element interposed between the clamp and the surface of the glass rod. In assembling the unit, the helix 21 is placed over the core winding assembly and one end secured to the core by the clamp 25. Then the slack is taken up until the glass helix sits firmly on the turns of the resistance element 12 and the free end secured to the core by the clamp 26. v

The construction embodying the invention has impor tant advantages, notably the capability to withstand high ambient temperatures and prolongation of the useful life of the potentiometer during which the electrical characteristics of the unit are not materially affected.

While borosilicate glass has been found to be satisfactory for the glass coil, other glasses and dimensionally stable glass-like material, such as quartz, may also be used.

It will be evident that the use of a high-temperature insulating member of this character is not limited to the formation of a contact track or guide but may also be used as an insulating or spacing member Where the properties of glass render it desirable for such uses. Furthermore while borosilicate glass has been found particularly suitable in carrying out the invention, other similar insulating materials may be used, as stated above. Thus the above-described potentiometer construction is merely illustrative of the application of the principles of the invention and numerous other arrangements may be devised by those skilled in the art without departing from the spirit and scope of the invention.

What is claimed is:

1. In a multiturn precision potentiometer comprising a helical resistance element, an insulating support for said resistance element, a movable contact member engaging said element, means for supporting said contact member for rotation around the longitudinal axis of said helical resistance element, and means including a rotatable input shaft for driving said contact member to adjust the point of contact along the length of said resistance element, the improvement comprising a preformed helical glass guide member for said contact member, said guide member consisting of a solid glass rod conforming to the shape of the surface of said resistance element and mounted in a position to define a helical groove the bottom of which is bounded by said element.

2. The method of making a resistance element for a multiturn precision potentiometer comprising the steps of forming a resistance element in a multitur-n coil, and

mounting a preformed glass coil on the convolutions of said resistance element to form an elongated glass boundary surface along the length of said element.

References Cited UNITED STATES PATENTS 2,815,422 12/1957 Lock 338-146 3,119,088 1/1964 Fleigler 338143 FOREIGN PATENTS 759,496 3/1953 Germany.

RICHARD M. WOOD, Primary Examiner.

I. G. SMITH, Assistant Examiner. 

1. IN A MULTITURN PRECISION POTENTIOMETER COMPRISING A HELICAL RESISTANCE ELEMENT, AN INSULATING SUPPORT FOR SAID RESISTANCE ELEMENT, A MOVABLE CONTACT MEMBER ENGAGING SAID ELEMENT, MEANS FOR SUPPORTING SAID CONTACT MEMBER FOR ROTATION AROUND THE LONGITUDINAL AXIS OF SAID HELICAL RESISTANCE ELEMENT, AND MEANS INCLUDING A ROTATABLE INPUT SHAFT FOR DRIVING SAID CONTACT MEMBER TO ADJUST THE POINT OF CONTACT ALONG THE LENGTH OF SAID RESISTANCE ELEMENT, THE IMPROVEMENT COMPRISING A PREFORMED HELICAL GLASS GUIDE MEMBER FOR SAID CONTACT MEMBER, SAID GUIDE MEMBER CONSISTING OF A SOLID GLASS ROD CONFORMING TO THE SHAPE OF THE SURFACE OF SAID RESISTANCE ELEMENT AND MOUNTED IN A POSITION TO DEFINE A HELICAL GROOVE THE BOTTOM OF WHICH IS BOUNDED BY SAID ELEMENT. 